[vm] Avoid embedding sources in CFE compilations if we AppJit

Creation of AppJit snapshot will not include the source of [Script]
objects (see `UntaggedScript::snapshot_to`). As a result there's no
point in letting the CFE embed the sources into the kernel if we
only use it to create an AppJit snapshot.

This is also in line with build rules for our dart-sdk, where we do the
kernel compilation separately, see utils/application_snapshot.gni:

```
  template("application_snapshot") {
    ...
    # Build the kernel file using the prebuilt VM to speed up the
    # debug and simulator builds.
    prebuilt_dart_action(target_name + "_dill") {
      ...

      args = [
        ...
        "--no-embed-sources",
        ...
      ]
      [[[
    }
    ...
  }
```

TEST=ci

Change-Id: I4e17e49dc21af6102d62c2278dbd6ebbe387f7e8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/313560
Reviewed-by: Slava Egorov <vegorov@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>